The Alpine Fault

The Alpine Fault system in New Zealand (Fig. 8.2a) provides an example of a continental transform whose structure reflects a large component of fault-perpendicular shortening. Geophysical observations of the sea floor south of New Zealand suggest that contraction originated with changes in the relative motion between the Australian and Pacific plates between 11 and 6 Ma (Walcott, 1998; Cande & Stock, 2004). Prior to ~11 Ma, relative plate motion resulted in mostly strike-slip movement on the Alpine Fault with a small component of fault-perpendicular shortening. After ~11 Ma and again after ~6 Ma, changes in the relative motion between the Pacific and Australian plates resulted in an increased component of compression across the pre-existing Alpine Fault, and led to increased shortening and rapid uplift of the Southern Alps (Norris et al., 1990; Cande & Stock, 2004). The changes produced an oblique continent-continent collision on the South Island. In the central part of the island uplift rates range from 5 to 10 mm a-1 (Bull & Cooper, 1986) and are accompanied by high rates of erosion. Together with the crustal shortening, these processes have led to the exhumation of high grade schist that once resided at depths of 1525 km (Little et al., 2002; Koons et al., 2003).

The Alpine Fault crosses the South Island between the Puysegur subduction zone in the south and the Hikurangi subduction zone in the north (Fig. 8.2a). During the late Cenozoic, the fault increasingly became the locus of slip between the Australian and Pacific plates. Geodetic measurements (Beavan et al., 1999) and offset glacial deposits (Fig. 8.4) suggest that it has accommodated some 60-80% of relative plate motion since the late Pleistocene (Norris & Cooper, 2001; Sutherland et al., 2006). The remaining motion is accommodated by slip on dipping thrust and oblique-slip faults in a >100-km-wide zone located mostly to the east of the fault (Fig. 8.2a). Geologic reconstructions of basement units suggest that a total of 850 ± 100 km of dextral movement has accumulated along the plate boundary since about 45 Ma (Sutherland, 1999). At least 460 km of this motion has been accommodated by the Alpine Fault (Wellman, 1953; Sutherland, 1999), as indicated by the dextral offset of the Median Batholith (Fig. 8.2a) and other Mesozoic and Paleozoic belts. About 100 km of shortening has occurred across the South Island since ~10 Ma (Walcott, 1998).

The subsurface structure of the Alpine Fault beneath the central South Island differs from that displayed by strike-slip-dominated transforms, such as the San Andreas and Dead Sea faults. Seismic imaging (Davey et al., 1995) indicates that the central segment of the Alpine Fault dips southeastward at angles of 40-50° to a depth in excess of 25 km (Fig. 8.2b). Motion on the fault is in a direction that plunges approximately 22°, indicating that the fault in this region is an oblique thrust (Norris et al., 1990). By contrast, motion on the Fiordland segment of the fault is almost purely strike-slip (Barnes et al., 2005).

A 600-km-long seismic velocity profile, constructed as part of the South Island Geophysical Transect (SIGHT), has revealed the presence of a large crustal root beneath the Southern Alps (Fig. 8.2b). On the Pacific side, the Moho deepens from ~20 km beneath the Canterbury Plain to a maximum depth of 37 km below a point located 45 km southeast of the surface trace of the Alpine Fault. The root is asymmetric and mimics the tapered profile of the Southern Alps at the surface: Moho depths southeast of the fault decrease more gradually than those on its northwest side (Scherwath et al., 2003; Henrys et al., 2004). The root is composed mostly of thickened upper crust with seismic velocities ranging between 5.7 and 6.2 km s-1 (Scherwath et al., 2003; Van Avendonk et al., 2004). At large distances from the plate boundary, the upper crust shows a normal thickness of ~15 km. A thin (3-5 km) lower crust with a velocity range of 6.5-7.1 km s-1 occurs at the base of the root. A low velocity zone occurs in the middle and lower crust below the fault trace, most likely as a result of high fluid pressure (Section 8.6.3) (Stern et al., 2001, 2002), and extends downward into the upper mantle.

Below the crustal root, teleseismic data show that deformation becomes progressively wider with depth. Measurements of Pn wave speeds (Scherwath et al., 2002; Baldock & Stern, 2005) and shear wave (SKS) splitting (Klosko et al., 1999; Duclos et al., 2005) suggest the presence of a zone of distributed ductile deformation in the upper mantle beneath the Alpine Fault. Fast polarization directions generally are oriented subparallel to the fault strike (Fig. 8.15), suggesting flow parallel to the plate boundary. Baldock & Stern (2005) found evidence for two distinctive domains beneath the South Island: a 335-km-wide zone of mantle deformation in the south and a narrower, ~200-km-wide zone in the north (Fig. 8.15). These widths and the orientation of the mantle anisotropy are consistent with a model of transpression involving 800 ± 200 km of right lateral strike-slip displacement, which is close to that predicted by geologic reconstructions.

Figure 8.15 Map showing the geometry of the SIGHT experiment and SKS measurements with an interpretation of mantle deformation below the Alpine Fault (AF) (image provided by T. Stern and modified from Baldock & Stern, 2005, with permission from the Geological Society of America). Three seismic transects (Tl, T2, T3) are shown. Black bars indicate direction of maximum seismic velocity. Bar length is proportional to the amplitude of shear wave splitting determined from the SKS results of Klosko et al. (1999). Pn anisotropy measurement of 11.5 ± 2.4% is from Scherwath et al. (2002).

Figure 8.15 Map showing the geometry of the SIGHT experiment and SKS measurements with an interpretation of mantle deformation below the Alpine Fault (AF) (image provided by T. Stern and modified from Baldock & Stern, 2005, with permission from the Geological Society of America). Three seismic transects (Tl, T2, T3) are shown. Black bars indicate direction of maximum seismic velocity. Bar length is proportional to the amplitude of shear wave splitting determined from the SKS results of Klosko et al. (1999). Pn anisotropy measurement of 11.5 ± 2.4% is from Scherwath et al. (2002).

100 km

100 km

Figure 8.16 Cartoons showing two possible modes of convergence in the mantle below the Alpine Fault (after Stern et al., 2002). (a) Symmetric root formed by homogeneous shortening and thickening. (b) Westward underthrusting of Pacific mantle lithosphere beneath the Australian plate forming a zone of intracontinental subduction.

Crust

Mantle Lithosphere

Mantle Lithosphere

Figure 8.16 Cartoons showing two possible modes of convergence in the mantle below the Alpine Fault (after Stern et al., 2002). (a) Symmetric root formed by homogeneous shortening and thickening. (b) Westward underthrusting of Pacific mantle lithosphere beneath the Australian plate forming a zone of intracontinental subduction.

The vertical thickness of the mantle root beneath the South Island is at least 100 km (Stern et al., 2002). Earthquakes occur between 30 and 70 km depth (Kohler & Eberhart-Phillips, 2003). The root has a core of relatively cool, dense, high velocity mantle lithosphere that has been displaced into hotter, less dense, slower asthe-nosphere (Scherwath et al., 2006). This excess mass in the mantle is required by observed gravity anomalies and provides sufficient force to maintain the crustal root, which is twice as thick as necessary to support the topography of the Southern Alps (Stern et al., 2000). One possible interpretation of the root geometry is that it is symmetric and has formed in response to distributed deformation and a uniform thickening of the lithosphere (Fig. 8.16a). Alternatively, the mantle root may be asymmetric, requiring the deformation to be concentrated on a dipping thrust surface that results from intracontinental subduction (Fig. 8.16b). These and other processes that contribute to mantle root formation and its tectonic modification are key elements of studies in virtually all zones of continental deformation (e.g. Sections 7.5, 7.8.1, 10.2.5, 10.4.6), and are discussed in more detail in Section 11.3.3. Whichever of these hypotheses is correct, the anomaly suggests that the low upper mantle temperatures beneath the Southern Alps are caused by cold downwelling beneath the collision zone. In addition, the teleseismic data indicate that the displacements associated with continental transforms can be accommodated by distributed deformation in the mantle without requiring discrete faulting. The great width of the deforming zone found in the New Zealand setting compared to other continental transforms may reflect the large component of convergence across the plate boundary (Stern et al., 2002).

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